Recognition that anthropogenic carbon dioxide (CO2) input to the atmosphere has substantially increased atmospheric carbon dioxide concentration, coupled with the theory that increased carbon dioxide, in turn, could drive rapid global warming, has focused attention on techniques—known as carbon sequestration—to remove some carbon dioxide from the atmosphere. One possibility is the conversion of carbon dioxide gas to stable, solid carbonate minerals such as calcite (CaCO3) and magnesite (MgCO3).
Because of its high concentration of Mg, tectonically exposed peridotite from the Earth's upper mantle, which is composed largely of the mineral olivine ((Mg,Fe)2SiO4), with lesser proportions of pyroxene minerals ((Mg,Fe,Ca)2Si2O6) and spinel ((Mg,Fe)(Cr,Al)2O4), and its hydrous alteration product serpentine, have been considered a promising reactant for conversion of atmospheric carbon dioxide to solid carbonate.
Natural carbonation of peridotite has been found to be surprisingly rapid. For example, carbonate veins in mantle peridotite in Oman have an average 14C age of approximately 26,000 years, and are not 30 to 95 million years old as previously believed. These data and reconnaissance mapping show that approximately 104 to 105 tons per year of atmospheric carbon dioxide are converted to solid carbonate minerals via peridotite weathering in Oman. Mantle peridotite is ordinarily more than 6 km below the seafloor, and is strongly out of equilibrium with air and water at the Earth's surface. Its exposure along large thrust faults and along tectonic plate boundaries creates a large reservoir of chemical potential energy.
Despite the available chemical potential, engineering techniques for carbon sequestration have many challenges. Engineering solutions involve grinding peridotite to a fine powder, purifying carbon dioxide gas, using reaction vessels at elevated pressure, and/or heating reactants to 100 degrees Celsius or more, and this comes at a substantial financial and energy cost.
Various methods have been experimentally evaluated, including direct carbonation of olivine and serpentine powders at elevated temperature, dissolution of serpentine or olivine in hydrochloric acid followed by carbonation of Mg and Ca ions in solution, and reaction of olivine and serpentine with carbonic acid (very similar to natural alteration). Reaction kinetics have been found to be too slow for significant sequestration of carbon dioxide unless the olivine or serpentine reactants are raised to more than 100 degrees Celsius, ground to a fine powder, and/or pre-treated at >600 degrees Celsius to increase reactive surface area. For the most part, because of the requirements for heating and processing, these approaches have been found to be too expensive—in financial terms and, more importantly, in energy expenditure—to be commercially viable at present.